The almost failsafe short of it. Use "alsamixer" to boost all recording volumes on main pulse and all cards (e.g. "alsamixer -c 0"). Start Audacity, edit -> preferences, stay in the "device" submenu, don't bother with the "recording" submenu. For each of the "Hosts" (alsa/jack), try all "Device"s under the "Recording" tab, start speaking, notice volume (or not and try the next one)
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